Conventionally, in an HEV, when an output from an engine is large with respect to the motive power required for driving, an electric generator is driven with surplus motive power to charge a secondary battery. On the other hand, when an output from an engine is small, a motor is driven with the electric power from a secondary battery to output supplementary motive power. In this case, a secondary battery is discharged. When a secondary battery is mounted on an HEV or the like, it is necessary to maintain an appropriate operation state by controlling such charging/discharging, etc.
In this case, it is important in battery control to perform output control for limiting the electric power supplied from a secondary battery. The output control aims at preventing damage and deterioration of the battery and increasing its life by transmitting information about the maximum output that can be discharged from the secondary battery, from an electronic control unit for a battery (hereinafter, abbreviated as a “battery ECU”) to an electronic control unit for a vehicle (hereinafter, abbreviated as a “vehicle ECU”).
The maximum output is determined in view of the balance between the electric power required by the vehicle for the battery and the holding time with the electric power. Both factors are important since they have an effect on drivability during driving.
Generally, the minimum voltage (discharging termination voltage) of the battery is set, and an output is limited when the minimum voltage is reached due to discharging. FIG. 4 shows an example of a change with the passage of time in a battery voltage Vb (voltage per block) and an output limit value Pout in a conventional output control method. In FIG. 4, the discharging termination voltage is set to 12.0 V, and the maximum output is limited to 20 kW. When the battery voltage Vb is decreased by discharging to reach 13.0 V as an output limit preparatory voltage in a normal output mode (time interval T1), a preparatory stage (time interval T2: about 10 seconds, for example) for an output limit is started. When the battery voltage Vb further is decreased to reach 12.0 V as the discharging termination voltage, an output limit mode (time interval T3) is started, where the output power Pout gradually is decreased (e.g., maximum decrease of 2 kW/second in rated output in FIG. 4) every time the battery voltage Vb reaches the discharging termination voltage (12.0 V).
However, when the holding time with the maximum output is made longer, the maximum output value becomes lower. Consequently, the burden on the engine of a vehicle side is increased in the case of an HEV or the like. On the other hand, when the holding time is made shorter, a high output can be realized in a moment. However, the battery voltage is decreased and an output is limited immediately, resulting in a drastic decline in drivability.
Steady driving is given top priority in vehicle control. Thus, the battery ECU has to transmit normal output information for longer holding time to the vehicle ECU rather than short-time output information for a high output only for a short time (e.g., several seconds or less).